This invention relates to a circuit for automatically controlling the signal level of first and second signals, particularly, first and second signals representative of a binary image of a target surface where the image is generated by a light beam scanning apparatus.
Equipment exists for automatic inspection of printed circuit boards which boards comprise a pattern of metallic conductors on a non-conducting or insulating substrate. In such equipment, a scanning light beam, such as a laser beam, periodically sweeps across the surface of the printed circuit board, and a detector assembly portion of the equipment detects the response of various portions of the surface to the impinging laser beam. For example, in U.S. Pat. No. 4,152,723, which is hereby incorporated by reference as if specifically set forth herein, when a scanning laser beam impinges on the substrate, it excites "a detectable fluorescence in the surface of the insulating substrate" which is "detected by means sensitive to the wavelength of the fluorescence." The detector means will generate first and second signals depending on the absence or presence of fluorescence in the yellow-red wavelength range which first and second signals represent the presence of conductor or substrate. When the first and second signals are digitized and sychronized with the scanning of the laser beam, a binary image of the pattern of the conductors on the substrate is provided. In order to form an accurate binary image, signals from the detector portion representing conductor (for example first signals) and signals from the detector portion representing substrate (for example second signals) must be distinguished from one another and identified as first or second signals for each printed circuit board inspected by the apparatus. However, different printed circuit boards having different conductor and substrate characteristics generate varying signal levels for the first and second signals from one printed circuit board to the next. For example, the signal levels due to fluorescence of the substrate in one printed circuit board may be quite a bit different from the signal levels due to fluorescence in the substrate in another printed circuit board from a different manufacturer. Also, the reflected light characteristics from the conductors on a first printed circuit board and their effect on the detector output may differ from the reflected light characteristics from the conductors on a second printed circuit board. Therefore, it is necessary to provide a method and apparatus for automatically controlling over a wide range the level of first and second signals received from the detector portion to insure that signals representing conductor and signals representing substrate will be identified and digitized as such to provide an accurate binary signal image of the printed circuit board.